When preparing OE devices like OFETs or OPV cells, in particular flexible devices, usually printing or coating techniques like inkjet printing, roll to roll printing, slot dye coating or flexographic/gravure printing are used to apply the OSC layer. Based on low solubility of the most of the present organic compounds useful as OSC these techniques need the use of solvents in high amounts.
In order to improve the film forming ability binding agents or wetting agents can be used. These additives are especially needed with regard to light emitting materials and/or charge transporting materials having small molecular weight or polymeric compounds having a low molecular weight.
However, these additives may impart drawbacks to the composition and/or the OE devices obtained thereof.
US 2007/0221885 relates to 3 solvent systems and is specifically relating to one solvent used to solubilise the active material, a second solvent that has a similar surface energy to the substrate and a third solvent which is used to make the other two solvents miscible. 3,4-dimethylanisole is mentioned within a list of solvents as a potential material to be used, it is not exemplified as one of the solvents. The main drive for this patent is to obtain printability onto low surface energy substrates.
WO 2006/122732 relates compositions, especially solutions, of at least one organic semiconductor emitting light from the triplet state, in an organic solvent or solvent mixture. The solvent may include 3,4-dimethylanisole. However, no example is provided. Furthermore, the document relates to polymeric organic semiconducting compounds.
The prior art provides compositions being useful in order to process low molecular weight organic light emitting and charge transporting materials. However, it is a permanent desire to improve the performance of the OLED layer, such as efficiency, lifetime and sensitivity regarding oxidation or water.
In addition thereto, the formulation should enable a low-cost and easy printing process. The printing process should allow a high quality printing at high speed.
It is therefore desirable to have improved formulations comprising an OSC that are suitable for the preparation of OE devices, especially thin film transistors, diodes, OLED displays and OPV cells, which allow the manufacture of high efficient OE devices having a high performance, a long lifetime and a low sensitivity against water or oxidation. One aim of the present invention is to provide such improved formulations. Another aim is to provide improved methods of preparing an OE device from such formulations. Another aim is to provide improved OE devices obtained from such formulations and methods. Further aims are immediately evident to the person skilled in the art from the following description.
Surprisingly it has been found that these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OE device using a formulation of the present invention.